Ethynylene-Linked Donor−Acceptor Alternating CopolymersWade A. Braunecker,*,† Stefan D. Oosterhout,† Zbyslaw R. Owczarczyk,† Ross E. Larsen,†
Bryon W. Larson,‡ David S. Ginley,† Olga V. Boltalina,‡ Steven H. Strauss,‡ Nikos Kopidakis,†
and Dana C. Olson†
†National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States‡Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States
*S Supporting Information
ABSTRACT: Controlling steric interactions between neighboring repeatunits in donor−acceptor (D−A) alternating copolymers can positivelyimpact morphologies and intermolecular electronic interactions necessaryto obtain high performances in organic photovoltaic (OPV) devices.Herein, we design and synthesize 12 new conjugated D−A copolymers,employing ethynylene linkages for this control. We explore D−Acombinations of fluorene, benzodithiophene, and diketopyrrolopyrrolewith analogues of pyromellitic diimide, thienoisoindoledione, isothianaph-thene, thienopyrazine, and thienopyrroledione. Computational modelingsuggests the ethynylene-containing polymers can adopt virtually planarconformations, while many of the analogous polyarylenes lacking theethynylene linkage are predicted to have quite twisted backbones (>35°).The introduction of ethynylene linkages into these D−A systemsuniversally results in a significant blue-shift in the absorbance spectra (by as much as 100 nm) and a deeper HOMO value(∼0.1 eV) as compared to the polyarylene analogues. The contactless time-resolved microwave conductivity technique is used tomeasure the photoconductance of polymer/fullerene blends and is further discussed as a tool for screening potential active layermaterials for OPV devices. Finally, we demonstrate that an ethynylene-linked alternating copolymer of diketopyrrolopyrrole andthienopyrroledione, with a rather deep LUMO estimated at −4.2 eV, shows increased photoconductance when blended with aperfluoroalkyl fullerene C60(CF3)2 as compared to the standard PC61BM. We attribute the change in increased free carriergeneration to the higher electron affinity of C60(CF3)2 that is more appropriately matched with the deeper LUMO of thepolymer.
■ INTRODUCTION
Conjugated polymers have been used extensively in recentyears for a number of organic electronic applications, includinglight-emitting diodes,1 field effect transistors,2 and organicphotovoltaic (OPV) cells.3−5 Through the design andmodification of the individual electron donating (D) andaccepting (A) components in alternating conjugated D−Acopolymers,6,7 intramolecular charge transfer within thesesystems can be readily manipulated with endless permutationsto fine-tune the electronic and optoelectronic properties of thepolymers for specific applications. In the field of OPV, thisstrategy has enabled the development of new materials withhighly tailored band gaps having both improved open circuitvoltages and improved capacity for harvesting photons from thesolar spectrum.8 OPV technology has begun to find commercialapplication,9 but further systematic research will be necessary todrive device efficiencies higher before adoption of thetechnology is realized on a large industrial scale.The successful design of a D−A copolymer for OPV
applications must take into account certain steric andconformational constraints. Repulsive steric interactionsbetween neighboring aromatic repeat units can impart twisting
throughout a polymer backbone. Such twisting is generallyconsidered undesirable, as planarity can effectively increase thepolymer conjugation length in D−A systems and can alsofacilitate π−π stacking and/or crystalline packing of thepolymer chains, all of which can enhance intermolecularelectronic interactions critical to the function of an OPVdevice.10 When steric interactions between conjugated arylcomponents do impart twisting through a polymer, an alkynelinkage could in principle relieve that strain.With the increasing availability of efficient procedures
developed for palladium-catalyzed alkynylation reactions overthe past few decades,11−13 poly(arylene ethynylenes), alsoknown as poly(arylacetylenes), are finding broad application asa promising class of organic semiconductors.14,15 The use ofethynylene-based materials in OPV has recently beenreviewed.16 Ethynylene-containing materials have foundapplication in a number of small molecule,17,18 polymer,19−22
and metallopolyyne23−25 photovoltaic devices. While device
Received: February 1, 2013Revised: April 10, 2013Published: April 29, 2013
efficiencies generally have not exceeded 4%,26 the ethynylene-based OPV materials sometimes have advantages over theirfully cyclic analogues. For example, the open circuit voltage ofpolythiophene was improved by nearly 0.4 V upon theintroduction of alkyne linkages, which was attributed to theelectron-withdrawing nature of the ethynylene group thatsignificantly lowered the highest occupied molecular orbital(HOMO) of the polymer.21 Alkyne linkages were alsodemonstrated to systematically improve (red-shift) theabsorption spectrum of certain sterically constrained polyar-ylenes more than 100 nm, consistent with a more highlyconjugated and planar backbone.27 Thus, when used appropri-ately, the introduction of an alkyne linkage could be animportant tool for fine-tuning the electronic and optoelectronicproperties of certain OPV polymers. However, only a handfulof ethynylene-containing D−A copolymers have been reportedin the literature.22,28−32 Even fewer systematic studies haveprobed how the introduction of an ethynylene group into apolycyclic D−A system will affect intramolecular charge transferrelative to its fully heterocyclic analogue.30
This work details the design, synthesis, and characterizationof 12 new D−A copolymers containing alkyne linkages for anumber of different systematic studies. Several D−A copoly-mers are synthesized with and without alkyne linkages; theiroptical and electronic properties are characterized, and theeffect of the alkyne linkages on the photoconductance of thematerials is examined, as determined by time-resolved micro-wave conductivity (TRMC) experiments. Other D−A copoly-mers are predicted by computational modeling to be so twistedwithout alkyne linkages that only the triple-bond-basedanalogues were synthesized and characterized. Furthermore,after designing and synthesizing some new “electron deficient”low band gap materials by linking traditional electron-withdrawing comonomers together with electron-withdrawingethynylene, we illustrate how the photoconductance ofmaterials with rather deep LUMO levels can be improvedover systems with PC61BM by appropriately matching thepolymers with the higher electron affinity fullerene C60(CF3)2,which has a deeper LUMO.
■ BACKGROUND AND DESIGN OF NEW MATERIALSIn a recent study, our group synthesized a D−A copolymer ofbenzodithiophene (BDT) and thienoisoindoledione (TID) (P-TID-BDT in Figure 1).33 Intramolecular charge transferbetween the alternating units of electron-donating andelectron-accepting moieties in this copolymer effectivelypromoted conjugation throughout its backbone by stabilizingthe polymer in its quinoidal state. Additionally, the fusedaromatic ring in TID provides further stabilization of thequinoidal state of the polymer, as the dearomatization of thethiophene ring to assume a quinoid structure is accompanied bya gain in aromatic resonance energy in the fused benzene ring.The resulting band gap of P-TID-BDT was thus appreciablylower (by ∼0.4 eV) than in an analogous copolymer ofthienopyrroledione (TPD) and BDT that was not stabilized bysuch aromatic resonance. However, despite the optimized bandgap, P-TID-BDT displayed lower photoconductance, asdetermined by TRMC, and decreased device efficiency (2.1%vs 4.8%) as compared with the TPD analogue.33 These resultswere partially attributed to morphology, as computationalmodeling suggested the TID copolymers would have a twistedbackbone, and X-ray diffraction data indicated the polymerfilms did not form ordered domains, whereas TPD copolymers
were considerably more planar and were shown to formpartially ordered domains.In an effort to improve the geometry of P-TID-BDT, and in
turn the morphology and photoconductance of the material, wedesigned an analogous D−A copolymer with alkyne linkages(P-TID-≡-BDT) as a follow-up to our original effort. Figure 2
illustrates the geometric structures for oligomers of P-TID-BDT and P-TID-≡-BDT, as calculated by density functionaltheory (DFT) and optimized in vacuum as described in theExperimental Section. As can be seen in the top of Figure 2 forP-TID-BDT, the hydrogen atom on the isoindole unit of TIDcreates sufficient steric hindrance with the neighboring BDTunit that a dihedral angle of ∼34° is induced between these
Figure 1. Benzodithiophene (BDT)-based polymers with thienoi-soindoledione (TID) and diketopyrrolopyrrole (DPP) cores.
Figure 2. Illustration of the twist in P-TID-BDT (top) and planarity ofP-TID-≡-BDT (middle and bottom).
comonomers, introducing significant twist throughout thepolymer backbone. However, the middle and bottom of Figure2 illustrate the relative planarity of P-TID-≡-BDT, with thedihedral angle being less than 4°.The planarity of P-TID-≡-BDT will in principle enhance
π−π stacking interactions. However, it is not straightforward topredict the impact of the ethynylene unit on the optoelectronicproperties. In a recent study of a twisted quinoxaline-basedhomopolymer, ethynylene linkages were introduced to allowthe polymer to adopt a more planar structure.27 The absorptionof the resultant polymer was red-shifted more than 100 nm ascompared with the original material, consistent with a morehighly conjugated and planar backbone. However, in anotherstudy concerning a benzothiadiazole-based D−A copolymerthat was relatively planar to begin with, the introduction of anethynylene linkage between the D−A components blue-shif tedthe absorption ∼100 nm relative to its fully cyclic analogue.30
The “push-pull” effect between donor and acceptor moietieswas apparently disrupted in the latter system by the ethynylenelinkage. Thus, it is not immediately clear the effect thatethynylene will have in P-TID-≡-BDT, as conjugation and π−πstacking interactions could be enhanced through planarizationof the twisted polymer backbone, yet intramolecular chargetransfer between TID and BDT could be negatively impactedby ethynylene. We further investigate the effect of ethynylenelinkages on D−A copolymers of diketopyrrolopyrrole (DPP)and BDT (P-DPP-BDT and P-DPP-≡-BDT in Figure 1). Theoptimized DFT structure for P-DPP-BDT shows a dihedralangle of 8°−19° (for anti and syn conformations, respectively)between comonomers. P-DPP-≡-BDT is significantly moreplanar (2°−6°).Next, a number of fluorene (FLR)-based D−A copolymers
were designed that incorporate ethynylene linkages, shown inFigure 3. A comparison of the optimized DFT structures ofFLR-based copolymers with analogous BDT-based systems
reveals that the FLR-based systems have greater torsion angles.For example, the copolymer P-TID-FLR is predicted to have adihedral angle of 41° (Figure S1 in the SupportingInformation) compared to 34° for P-TID-BDT. This can beattributed to the greater intrinsic steric constraints imposed onthe TID comonomer by the six-member ring in FLR comparedto the five-member ring in BDT. In the case of pyromelliticdiimide (PMDI) and FLR, the dihedral angle is as high as 57°without ethynylene linkages (Figure S2). Given the extremetwisting in these polymers, only analogues containingethynylene were synthesized (Figure 3). Such polymers arepredicted by DFT to be virtually planar (Figures S1 and S2).The optical/electronic properties and photoconductance of theFLR-based ethynylene-containing copolymers with TID,PMDI, and isothianaphthene (ITN) are compared with theDPP-containing copolymer P-DPP-≡-FLR (Figure 3), whichhas shown OPV device efficiencies of ∼2% in the literature.22
Finally, the incorporation of ethynylene linking unitsbetween electron-withdrawing comonomers opens a syntheticpathway for us to design rather electron-withdrawingalternating copolymers with narrow band gaps and relativelydeep HOMO values. For example, the bromine-functionalizedmonomer TID cannot be copolymerized in alternating fashionwith bromine-functionalized DPP to give P-TID-DPP.However, following the introduction of ethynylene onto theDPP core, the copolymer P-TID-≡-DPP can now besynthesized through a Sonogashira cross-coupling reaction.This relatively electron-withdrawing copolymer should have adeep HOMO level, and the aromatic resonance stabilizationafforded by the TID group should keep the band gap relativelynarrow. Six new copolymers of DPP with comonomers ofvarious electron withdrawing strength (TPD, PMDI, TID, ITN,DPP, and thienopyrazine (TP), Figure 4) were synthesized inan effort to provide materials with a range of energy levels forsystematic studies in OPV.
■ EXPERIMENTAL SECTIONGeneral. All reagents employed in this study were obtained from
commercial sources at the highest available purity and used withoutfurther purification, unless otherwise noted. All reactions wereperformed under dry N2. Methylene chloride, toluene, and THFwere purified by passing through alumina in an MBraun solventpurification system. Column chromatography was performed withFluka Silica Gel 60 (220−440 mesh). All small molecules werecharacterized by 1H NMR (400 MHz) and 13C NMR (100 MHz) on aVarian Unity Inova. Monomers were >99% pure as determined by 1HNMR. PCBM was purchased from Nano-C, Inc., was >99% pure andwas used without further purification. C60(CF3)2 was synthesizedaccording to the literature method34 and purified to >99% asdetermined by HPLC, 19F NMR, and APCI-MS, described in detailin the Supporting Information. UV−vis absorption measurementswere performed using a Hewlett-Packard 8453 UV−vis spectropho-tometer.
Polymer Molecular Weight Determination. Polymer sampleswere dissolved in HPLC grade chloroform (∼1 mg/mL), stirred andheated at 50 °C for 2 h, stirred overnight at rt, and then filteredthrough a 0.45 μm PVDF filter. Size exclusion chromatography wasthen performed on a PL-Gel 300 × 7.5 mm (5 μm) mixed D columnusing an Agilent 1200 series autosampler, inline degasser, and diodearray detector. The column and detector temperatures were 35 °C.HPLC grade chloroform was used as eluent (1 mL/min). Linearpolystyrene standards were used for calibration.
Cyclic Voltammetry. All voltammograms were recorded at 25 °Cwith a CH Instruments Model 600D potentiostat. Unless otherwisespecified, measurements were carried out under nitrogen at a scanningrate of 0.1 V s−1 using a platinum wire as the working electrode and a
platinum wire as the counter electrode. Potentials were measured vsAg/Ag+ (and calibrated vs Fc/Fc+) using 0.01 M AgClO4 and a 0.1 MBu4NBF4 salt bridge to minimize contamination of the analyte withAg+ ions. Polymer films were drop cast onto a platinum wire workingelectrode from a 1 mg/mL chloroform solution and dried under astream of nitrogen prior to measurement in a 0.1 M Bu4NBF4−acetonitrile solution.Theory. Density functional theory (DFT) was used to predict the
structural properties of the polymers reported in this work forhydrogen-terminated oligomers with n = 1−4. All calculations wereperformed with the default settings in the Gaussian 09 electronicstructure package, revision B.01.35 The geometric structure of eacholigomer was optimized in vacuum using the Becke-style three-parameter density functional with the Lee−Yang−Parr correlationfunction (B3LYP) with the 6-31G(d) basis set.Time-Resolved Microwave Conductivity. TRMC is a pump−
probe technique that can be used to measure the photoconductance ofa film without the need for charge collection at electrical contacts.36,37
The details of the experimental methodology have been presentedelsewhere.37,38 In brief, the sample is placed in a microwave cavity atthe end of an X-band waveguide operating at ca. 9 GHz and isphotoexcited through a grid with a 5 ns laser pulse from an OPO
pumped by the third harmonic of an Nd:YAG laser. The relativechange of the microwave power, P, in the cavity, due to abrorption ofthe microwaves by the photoinduced free electrons and holes, isrelated to the transient photoconductance, ΔG, by ΔP/P = −KΔG,where the calibration factor K is experimentally determinedindividually for each sample. Taking into account that the electronsand holes are generated in pairs, the peak photoconductance duringthe laser pulse can be expressed as37
∑β ϕ μΔ =G q F I ( )e A 0 (1)
where qe is the elementary charge, β = 2.2 is the geometric factor forthe X-band waveguide used, I0 is the incident photon flux, FA thefraction of light absorbed at the excitation wavelength, ϕ is thequantum efficiency of free carrier generation per photon absorbed, and∑μ is the sum of the mobilities of electrons and holes.37 Equation 1 isused to evaluate the quantum efficiency or free carrier generation perphoton absorbed and the local mobility of free carriers. Thesequantities can then be correlated to the molecular structure to provideinsight into the mechanisms for free carrier generation and transport inpolymer−fullerene composites as a function of the microstructure.The photoconductance decay after the end of the laser pulse is also a
Figure 4. Diketopyrrolopyrrole (DPP)-based polymers with thienopyrrolodione (TPD), pyromellitic diimide (PMDI), thienoisoindoledione (TID),isothianaphthene (ITN), DPP, and thienopyrazine (TP) cores.
Table 1. Number-Average Molecular Weight (Mn), Polydispersity Index (PDI), and Optical and Electrochemical Properties ofPolymersa
λmax, λ0.1max (nm)
polymer Mn (kDa) PDI solutiona film Egopt b (eV) EHOMO
aMeasured in chloroform solution; λ0.1max = wavelength at which absorption is 0.1 its maximum value. bCalculated from film λ0.1max.cEHOMO
estimated from onset potential measured at 0.1 V/s vs Ag/Ag+ and calibrated against Fc/Fc+ (measured as 0.1 V vs Ag/Ag+); Fc/Fc+ energy levelused in HOMO calculations was −4.8 eV.41
useful tool for the characterization of free carrier decay mechanisms byrecombination and trapping.
■ RESULTS AND DISCUSSION
Material Synthesis. Synthetic procedures for all monomersand their precursors are described in the SupportingInformation, as are full synthetic proceedures for the polymer-izations. The two non-ethynylene-containing copolymers in thisstudy, P-TID-BDT and P-DPP-BDT, were synthesized by apalladium-catalyzed Stille coupling at 110 °C over 36 h inchlorobenzene. After this time, the polymers were end-cappedwith thiophene reagents, as it has been demonstrated that suchend-capping can improve the performance of photovoltaicdevices.39 A palladium scavenger40 was then stirred with thepolymer to complex the catalyst before the polymer wasprecipitated into MeOH, after which the polymer was purifiedvia Soxhlet extraction for 12 h with MeOH and 2 h withacetone. The 12 ethynylene-containing polymers weresynthesized by a palladium-catalyzed Sonogashira cross-coupling reaction at 60 °C over 6 h. The polymers were end-capped with ethynyltrimethylsilane, after which the polymerwas purified in an analogous manner as described above.As can be seen in Table 1, most of the Sonogashira
polymerizations proceeded smoothly to high molecularweights; however, the lower molecular weights (Mn < 30kDa) of P-TPD-≡-DPP, P-ITN-≡-DPP, and P-DPP-≡-DPPcould generally be attributed to solubility issues, as thepolymers began to precipitate from solution during thereaction. However, P-TID-≡-BDT (with Mn ∼ 8.2 kDa)remained soluble during the polymerization reaction, butmolecular weights did not increase after 6 h. Furthermore, ifleft to react for greater than 24 h, the brilliant purple color ofthe polymer began fading to a gray-purple, indicating significantdecomposition of P-TID-≡-BDT under the longer reactiontimes; thus, all subsequent measurements on this polymer wereconducted on a sample obtained after 6 h.Optical Characterization of Polymers. The absorption
spectra of the polymers were measured both in solution(chloroform) and in the solid state as thin films. Severalimportant observations are worth noting. First, the introductionof ethynylene into these alternating copolymers universallyresulted in a blue-shift in the absorbance spectrum in bothsolution and the solid state. These results were not necessarilyanticipated, especially for P-TID-≡-BDT, given the recentliterature results that demonstrated the introduction ofethynylene into a twisted quinoxaline homopolymer red-shiftedabsorbance more than 100 nm in the resultant polymer,consistent with a more highly conjugated and planarstructure.27 Despite the more planar structure predicted forP-TID-≡-BDT (see Figure 2), λmax of the thin flim of P-TID-≡-BDT blue-shifts 79 nm relative to P-TID-BDT (Figure 5).The presence of the electron-withdrawing ethynylene groupthus seemingly interupts the “push−pull” effect between theBDT and TID units that dictates the optical band gap in thefully heterocyclic polymer analogue. For P-DPP-≡-BDT, theblue-shift in λmax of the film is not quite as dramatic with just a13 nm difference between P-DPP-≡-BDT and P-DPP-BDT(Figure 5). Similarly, λmax of the P-DPP-≡-FLR film blue-shiftsbetween 9 and 21 nm as compared to several literatureanalogues of P-DPP-FLR with different alkyl chains.42 For P-DPP-≡-DPP, the effect of ethynylene is substantial, as λ0.1maxblue-shifts ∼100 nm relative to the literature polymer P-DPP-DPP.42 The latter polymer was recently used as the active layer
in an OPV device, but only 0.3% device efficiency was obtained.This was attributed to the small LUMO−LUMO offset withPC61BM that was an insufficient driving force for chargeseparation in the solar cell device.The absorbance spectra of P-TID-≡-FLR and P-DPP-≡-FLR
are blue-shifted relative to their BDT containing analogues. Thethin film optical band gap of these FLR-containing polymersare 390 and 130 meV wider, respectively, than P-TID-≡-BDTand P-DPP-≡-BDT. This can likely be attributed to the weakerelectron-donating power of the FLR unit relative to BDT.Similar trends have been seen in the literature for non-ethynylene-containing D−A copolymers. For example, theoptical band gap we measured for P-DPP-BDT is 340 meVsmaller than that reported in the literature for P-DPP-FLR.42
The polymer with the largest band gap of the 14 polymersinvestigated in this study was P-PMDI-≡-FLR. This band gapof 2.30 eV results from the combination of the strong electron-withdrawing PMDI unit with the weak electron-donating FLRunit. Also, the three polymers in this study with the smallestband gaps, P-TID-≡-DPP, P-ITN-≡-DPP, and P-TP-≡-DPP, allemploy electron-withdrawing comonomers that contain fusedaromatic rings. As discussed earlier, it has been welldocumented that in such systems the dearomatization of thethiophene ring to assume a quinoid structure is stabilized bothby the push−pull effect of the D−A groups and by a gain inaromatic resonance energy in the fused benzene ring.33
Absorption of the acceptor-rich DPP-based polymers is alsoquite broad compared to their FLR-based analogues (comparethe solid state spectra of P-TID-≡-DPP and P-ITN-≡-DPPwith P-TID-≡-FLR and P-ITN-≡-FLR in Figure 6), allowingthe DPP-based polymers to potentially harvest photons from abroader range of the solar spectrum. All solution spectra arepresented in the Supporting Information.
Electrochemical Characterization of Polymers. Cyclicvoltammetry was utilized to evaluate the HOMO energy levelsof all the polymers. The data are summarized in Table 1. Giventhat the redox processes were generally irreversible (illustratedin the Supporting Information), EHOMO was estimated from theonset potential. Potentials were measured vs Ag/Ag+ andcalibrated against the ferrocene/ferrocenium couple (Fc/Fc+
measured as 0.1 V vs Ag/Ag+). The Fc/Fc+ energy level used inHOMO calculations was assumed to be −4.8 eV.41 Thus,EHOMO = −(ϕox + 4.7) eV, where ϕox is the oxidation onsetpotential of the sample vs Ag/Ag+. Polymer films were drop-cast onto a platinum electrode from solutions with uniformconcentrations (1 mg/mL). The data are summarized in Figure
Figure 5. Effect of ethynylene linkages on polymer film absorbance(normalized).
7, where the LUMO, as we define it in this paper, wasdetermined from the addition of the optical band gap to theHOMO.A couple of important observations can be made about the
energy level diagram in Figure 7. First, the introduction of theelectron-withdrawing ethynylene into a polymer lowers theHOMO level with respect to the fully heterocyclic analogue.The HOMO of P-TID-≡-BDT was measured as 0.1 eV deeperthan P-TID-BDT, as was the HOMO of P-DPP-≡-BDT vs P-
DPP-BDT (Figure 7). This is generally consistent withliterature observations for the introduction of ethynylene intopoly(3-hexylthiophene)21 and a D−A copolymer of dioxythio-phene and benzothiadiazole.30 The HOMO of both of thoseliterature polymers was lowered by ∼0.3 eV upon introductionof ethynylene.The HOMO of P-PMDI-≡-FLR is quite deep, approximately
−6.0 V, with the LUMO estimated at −3.7 eV. In a recentstudy of a series D−A ethynylene-containing copolymers based
Figure 6. Normalized absorbance spectra of polymer films. Fluorene-based materials (top panel) and DPP-based materials (bottom panel).
Figure 7. Optical band gaps (determined from λ0.1max) and HOMO levels (determined from CV) for polymer films.
on PMDI, the LUMO values of five different PMDI-basedpolymers were all approximately −3.6 eV.32 The PMDI unitdictated the LUMO, while the HOMO varied 0.8 eV dependingon the strength of the donating unit. Weak donating units had aHOMO around −5.9 eV. Thus, our estimation of the HOMOand LUMO for P-PMDI-≡-FLR, with the weakly donating FLRgroup, is fully consistent with literature observations for similarpolymers. The other FLR-containing polymers also haverelatively deep HOMO values. While this would translate tolarge desirable open circuit voltages in OPV devices, therelatively wide band gaps (>2 V) of these polymers make themless than optimal for OPV applications. These wide band gapsare present despite the aromatic resonance stabilizationafforded by the ITN and TID groups. Of the four FLR-containing polymers investigated in this work, only P-DPP-≡-FLR is promising for OPV applications given its band gapcloser to 1.7 eV.The band gaps of the new acceptor-rich DPP-containing
polymers are in a much more optimal range for OPV than theFLR polymers. However, the estimated energy levels indicatethat the LUMO values for these polymers (between −4.0 and−4.2 eV) are likely too low for use in OPV devices withtraditional fullerene acceptors (i.e., PC61BM, with a LUMOaround −4.2 eV4). Previous results already suggested that theLUMO of P-TID-BDT (−4.0 V) was too low to allow efficientcharge separation at the polymer−PC61BM interface.33,43 TheLUMOs of these six polymers are all estimated to lie deeperthan that of P-TID-BDT. Such polymers would need to be usedin conjunction with deeper LUMO fullerene acceptors if theywere to be successfully employed in OPV devices.Time-Resolved Microwave Conductivity (TRMC). Con-
tactless photoconductivity has previously been used by us33,37,44
and others36,45 to study the photophysics of free carriergeneration and decay in bulk heterojunctions of polymer donormaterials with PC61BM. Clear correlations are emergingbetween the magnitude of the photoconductivity measuredby TRMC and the performance of complete OPV devi-ces.33,45,46
In this paper, TRMC was used to evaluate the effect of thealkyne linkage on the photocarrier generation and decaydynamics. For this purpose, P-DPP-BDT and P-DPP-≡-BDTwere compared in pure films and in bulk heterojunctions with5% and 50% PC61BM loading by weight. The low, 5% PC61BMloading is used to evaluate the efficiency of free carriergeneration at the polymer−fullerene interface: assumingfullerenes are dispersed into the polymer matrix in this “dilute”blend, small or negligible contribution to the sum of the freecarrier mobilities, ∑μ, from the electron mobility is expectedallowing us to attribute the increase in ϕ∑μ from the purepolymer to an increase in the quantum yield for free carriergeneration per photon absorbed, ϕ, due to the presence of thePC61BM.The ϕ∑μ product for the P-DPP-BDT and P-DPP-≡-BDT
systems is shown in Figure 8. While ϕ∑μ was measured overan intensity range spanning ca. 5 orders of magnitude, in Figure8 we compare the magnitude of ϕ∑μ at an absorbed photonflux of 1013 photons/(cm2 pulse) for simplicity. However, thesame trend was observed throughout the excitation intensityrange (Figure S40). Structure simulations (discussed above)showed a twist of ca. 20° in the backbone of P-DPP-BDT and asignificanly more planar structure for P-DPP-≡-BDT. Theresults of Figure 8 show that planarizing the polymer backbonehas no effect on the magnitude of ϕ∑μ for the pure polymer or
the blends. We note that adding 5% PC61BM to P-DPP-BDTcauses a small increase of ϕ∑μ, contrary to what is observedwith polymers such as P3HT, where addition of 5% PC61BM byweight increases ϕ∑μ by a factor of 40.37 Furthermore, thephotoconductance decays of both P-DPP-BDT and P-DPP-≡-BDT are relatively fast (Figure S21), indicating significantcarrier loss during the 5 ns laser pulse that limits the density offree carriers observed with TRMC. This indicates the presenceof an inherent limitation to the creation of free carriers in thesesystems, which is not overcome by planarizing the polymer viathe addition of the alkyne linkage.TRMC was then used to test the films of seven additional
polymers: P-TID-BDT (excitation at 660 nm), P-PMDI-≡-FLR(480 nm), P-ITN-≡-FLR (530 nm), P-TID-≡-FLR (540 nm),P-DPP-≡-FLR (640 nm), P-TPD-≡-DPP (680 nm), and P-TID-≡-DPP (680 nm), blended with PC61BM (1:1 weightratios). Universally lower magnitudes of ϕ∑μ (by a factor 10−100) were observed as compared to high performance polymersreported previously.33,37 The results are illustrated in FigureS20. A recent study of ours correlated the photoconductance ofseveral polymers determined by TRMC with their performancein OPV devices.33 Among those polymers was P-TID-BDT, forwhich ϕ∑μ of P-TID-BDT:PC61BM was found to be lower bya factor of 7 than that of an analogous polymer P-TPD-BDT.Device efficiencies of P-TID-BDT have been reported between2.1% and 3.0%,33,43 while efficiencies for P-TPD-BDT havebeen reported between 4.8% and 6.8%,33,47,48 in goodagreement with the relative difference in ϕ∑μ for these twopolymers.33 The lower photoconductance and OPV perform-ance of P-TID-BDT was partially attributed to morphologicalissues, as P-TID-BDT has a quite twisted backbone and couldnot order very well;33 it was also partially attributed to the low-lying LUMO of P-TID-BDT with respect to the LUMO ofPC61BM.43 The introduction of ethynylene in theory gave theresultant polymer P-TID-≡-BDT a more planar structure; italso widened the band gap so that the LUMO was not so deep(see Figure 7). However, as discussed earlier in the MaterialSynthesis section, this polymer was not stable at long reactiontimes, and its potential partial decomposition during the
Figure 8. Product of the yield for free carrier generation ϕ and thesum of mobilities ∑μ of electrons and holes obtained from the peakphotoconductance at an absorbed photon flux, FAI0, of 10
13 photons/(cm2 pulse) for thin films of P-DPP-BDT and P-DPP-≡-BDT,photoexcited at 680 nm. Blue bar: pure polymers; red bar: blend with5 wt % PCBM; black bar: blend with 50 wt % PC61BM.
polymerization deters us from drawing any hard conclusionsabout its photoconductance data.It should be noted that of all the polymers in this study, P-
TID-BDT gave the highest ϕ∑μ signal. Thus, the remainingpolymers are not anticipated to produce OPV deviceefficiencies higher than 3.0% reported for this polymer in theliterature. Literature values for optimized device efficiencies ofP-DPP-BDT49 and P-DPP-≡-FLR22 are 2.8% and 2.3%,respectively, in good agreement with our TRMC data thatsuggest they should be under 3%. As discussed above, thepresence of ethynylene in P-DPP-≡-BDT has virtually no effecton the TRMC signal compared to the fully heterocyclicanalogue. Device efficiency is thus anticipated to be in the rangeof 2−3% for this polymer as well. As generally quite low TRMCsignals were obtained for the remainder of the polymers, noOPV devices were made from these new materials.It is possible that while ethynylene in principle allows these
polymers to adopt a planar structure that could improvepacking over their cyclic analogues, the huge bulky side chainsneeded to solubilize the polymers actually prevent them fromdoing so. A lack of ordering has previously been correlated withslower transport of photogenerated carriers,50 which in turnimplies higher recombination losses. A detailed study of theeffect of side-chain size on long-lived carrier generation inpush−pull polymers will be presented in an upcomingpublication.In addition to morphological effects limiting free carrier
generation, for the acceptor-rich polymers investigated byTRMC (namely, P-TPD-≡-DPP and P-TID-≡-DPP), the poorLUMO−LUMO offset with PC61BM likely contributes to a lowyield of free carrier generation. As mentioned earlier, theLUMO of P-TID-≡-DPP (−4.0 eV) is similar to the LUMO ofP-TID-BDT, which is already believed to be quite deep forapplication with PC61BM. The LUMO of P-TPD-≡-DPP (−4.2eV) is deeper yet. The other four DPP based polymers inFigure 7 with band gaps <1.4 all have similar low-lying LUMOs.The corresponding decrease in the free energy available for freecarrier generation by exciton dissociation at the polymer−fullerene interface may thus limit ϕ. To this end, the C60(CF3)2fullerene illustrated in Figure 9, whose LUMO is ∼0.1 eVdeeper than that of C60,
51 which is in turn deeper than PC61BM
by ∼0.1 eV,52 was blended with P-TPD-≡-DPP to probe theextent to which ϕ is limited by poor free carrier generation insuch systems.In Figure 9, the ϕ∑μ product for P-TPD-≡-DPP with 5 wt
% ratio of PC61BM and the same molar equivalent of C60(CF3)2is shown. For these measurements, a higher loading of fullerenewas not pursued since C60(CF3)2 forms coarse clusters at highloading and phase-separates from the polymer phase. Figure 9shows that the ϕ∑μ product for P-TPD-≡-DPP increases by afactor of 4 upon going to C60(CF3)2. This evidence suggeststhat ϕ may be limited by poor LUMO−LUMO offset in the P-TPD-≡-DPP system with PC61BM and can be improved withan appropriate fullerene.
■ CONCLUSIONS
The introduction of ethynylene linkages into these D−Asystems universally resulted in a significant blue-shift in theabsorbance spectra of the polymers (by as much as 100 nm)and a deeper HOMO value (∼0.1 eV) as compared to theirfully cyclic analogues. In principle, these linkages could be usedas a tool to fine-tune the polymer band gaps and HOMO valuesfor OPV applications. They also provide a synthetic route tocopolymerize electron-withdrawing monomers in alternatingfashion. We used the latter procedure to design a new class ofmaterials with deep HOMO levels and narrow band gaps. Thepolymers in this study generally displayed poor photo-conductance. This was partially attributed to the large bulkyside chains needed to solubilize the polymer chains that canpotentially disrupt packing. It was also attributed to the deepLUMO values of some of the polymers that are believed to betoo deep to promote efficient exciton dissociation withPC61BM. We demonstrated that the photoconductance ofsuch a polymer could be improved when blended with thestronger fullerene electron acceptor C60(CF3)2. The latter resultsuggests that the use of deeper LUMO fullerene accept-ors51,53,54 with some of these new low band gap polymersystems represents a promising direction for future OPVdevelopements.
■ ASSOCIATED CONTENT
*S Supporting InformationCyclic voltammograms, solution absorbance spectra, TRMCdata, geometric structures of some polymers, synthetic detailsfor monomers and polymers, and 1H NMR spectra of all newmonomers and polymers. This material is available free ofcharge via the Internet at http://pubs.acs.org.
NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by the U.S. Department of Energyunder Contract DE-AC36-08-GO28308 with the NationalRenewable Energy Laboratory through the DOE SETPprogram and by the NSF (Grant CHE-1012468 to S.H.S.and O.V.B.).
Figure 9. Molecular structures of fullerenes used in TRMCexperiments and magnitude of the ϕ∑μ product for blends P-TPD-≡-DPP with either 5 wt % PC61BM (blue bar) or the molar equivalentof C60(CF3)2 (red bar) at an absorbed photon flux of ∼1014 photons/(cm2 pulse).
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